Solar collector and method

ABSTRACT

An apparatus and method for collecting solar energy are provided. The apparatus is a trough-type solar collector having one or more mirrors and lenses for directing solar radiation toward a receiver configured to receive a heat transfer fluid therein. The amount of solar radiation directed toward the receiver can be controlled by adjusting one or more of the mirrors and/or lenses or by adjusting a shade. Thus, the collector can direct different amounts or solar radiation toward the receiver, thereby selectively heating the receiver at different rates, e.g., to preheat the receiver, to heat fluid in the receiver for power generation, or to thaw solidified fluid in the receiver. Subsequently, the heated fluid can be used to generate steam and/or electricity.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to the solar energy devices, and, inparticular, relates to a trough solar collector that can be used tocollect solar energy for power generation.

2) Description of Related Art

In a conventional trough-type solar power generation system 10, asillustrated in FIG. 1, a plurality of parabolic collector mirrors 20 areused to reflect sunlight toward receivers 18 in which a heat transferfluid is circulated. The fluid, which can be organic or synthetic oil,is circulated by a pump 12 through a fluid circuit 14 that includes thereceivers 18 and a plurality of connection pipes 16. After being heatedby the collector mirrors 20, the fluid is delivered to a steam generator22, in which thermal energy is exchanged from the heat transfer fluid towater in a separate fluid circuit 24. Thus, the heat transfer fluid iscooled in the steam generator 22 and can then be re-circulated to thereceivers 18 for reheating. Small storage tanks (not shown) may also beincluded to store the heat transfer fluid. The water heated in the steamgenerator 22 forms steam that is circulated to a turbine generator 26,i.e., a turbine 28 coupled to an electrical generator 30. The steamexpands and rotates the turbine 28 and the generator 30, thus producingelectricity. The steam can be passed through a condenser 32 that, inconjunction with a cooling tower 34, condenses the steam to form hotwater that is preheated through preheater 33 and can be circulated backto the steam generator 22 by a pump 36 for re-use.

The parabolic collector mirrors 20, shown in FIGS. 2 and 3, typicallycan be pivoted such that each mirror 20 can be rotated according to arelative position of the sun 50. For example, a motor and drive 40 canrotate the mirrors 20 about the receivers 18. Thus, each mirror 20 cantypically be turned “on” by rotating the mirror 20 about the receiver 18so that the mirror 20 is directed normal to the sun 50 to collect andreflect solar radiation, as shown in FIG. 3. If not directedsubstantially normal to the sun 50, the mirror is in an “off” position.Due to the parabolic shape, the mirrors 20 do not heat the receivers 18unless rotated to the on position, where the receivers 18 are in thefocal line of the mirrors 20, and a small deviation in rotationalposition from the on position results in little or no heating of thereceivers 18.

At times of relative darkness, such as at nighttime and during cloudy orotherwise overcast weather conditions, sunlight cannot be used forheating the fluid. The circulation of the fluid is typically continuedto avoid problems associated with shutdown and/or startup of the system10. In particular, circulation can be continued to prevent thermalstresses due to temperature mismatch associated with introducing thefluid into the system 10 at a temperature that differs significantlyfrom the temperature of the receivers 18, and other components of thesystem 10, as can occur during startup. However, if the fluid iscontinuously circulated when the system 10 is receiving little or nosunlight, the fluid loses thermal energy to the cooler ambientenvironment. Further, if the temperature of the fluid falls below itsfreezing point, the fluid will solidify in the receivers 18 and/or thepipes 16. Uniform thawing of the fluid in the fluid circuit 14 can bedifficult, and the expansion of the fluid associated with freezingand/or thawing can over-stress, plastically deform, and eventually burstor otherwise destroy the receivers 18, and other components of thesystem 10. Therefore, in order to maintain the proper temperature of thefluid, the fluid is heated by electric or gas heaters as the fluidcirculates around circuit 14, increasing the energy required foroperating the system 10, reducing the overall efficiency, and addingpollutants to the environment.

As noted above, the collector mirrors 20 of the trough-type system 10are configured either in an “on” position in which solar energy isreflected at a maximum rate to the receivers 18 or in an “off” positionin which substantially no solar energy is reflected to the receivers 18.Therefore, if the receivers 18 are emptied at the end of each day andrefilled at the start of each subsequent day, it can be difficult usingthe system 20 to match the temperature of the fluid and the receivers 18during the daily filling operation.

Thus, there exists a need for a solar energy device that can accommodateand heat different types of heat transfer fluids, including those withvaried ranges of operating temperature. Preferably, the device should becapable of preheating and/or thawing the heat transfer fluid so that thefluid does not need to be circulated and/or heated continuously duringperiods of limited sunlight.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for collectingsolar energy. The apparatus is a trough-type solar collector having oneor more mirrors and/or lenses for directing and concentrating solarradiation toward a receiver configured to receive a heat transfer fluidtherein. The amount of solar radiation directed toward the receiver canbe controlled by adjusting one or more of the mirrors and/or lenses orby adjusting a shade. Thus, the apparatus can direct different amountsof solar radiation toward the receiver, thereby selectively heating thereceiver at different rates during similar solar conditions, e.g., topreheat the empty receiver, to heat fluid in the receiver for powergeneration, or to thaw solidified fluid in the receiver.

According to one embodiment of the present invention, the apparatusincludes a receiver extending along an axis and defining a passage forreceiving a heat transfer fluid. The receiver can be heated with solarradiation that is reflected and focused thereto by a first concavecollector mirror extending generally parallel to the receiver, e.g.,with the receiver located in the focal line of the mirror. Further, anoptical lens or a second mirror positioned adjacent the first mirror andextending generally parallel to the first mirror can be configured todirect solar radiation toward the receiver. Thus, the mirror and lensare configured to direct different amounts of solar radiation toward thereceiver such that the receiver can be heated at more than one rate. Thefirst mirror can be parabolic and/or rotatable so that the mirror canreceive solar radiation from various directions and direct the solarradiation toward the receiver. The second mirror or the lens can berigidly attached or adjustable relative to the first mirror. The lenscan be, for example, a fresnel lens structured to at least partiallyrefract light passing therethrough.

In embodiments that include a lens, the lens is adjustable between firstand second positions. For example, the lens can be fixedly positionedand configured to direct solar radiation from a plurality of directionstoward the receiver, and an adjustable shade can be provided tocontrollably prevent the transmission of solar radiation toward thereceiver and thereby control the amount of heating of the receiver.Alternatively, the lens can be rotatable about a longitudinal axis ofthe receiver so that the lens can direct solar energy from the suntoward the receiver throughout a range of relative positions of the sun.In the first position the lens directs solar radiation toward thereceiver to thereby heat the heat transfer fluid in the passage at afirst rate, and in the second position the lens directs a lesser amountof solar radiation toward the receiver than in the first position tothereby heat the receiver at a second rate less than the first rate.

The present invention also includes a method of heating a heat transferfluid using solar energy. The method includes circulating the heattransfer fluid through a receiver defining a passage. A solar collectionapparatus is adjusted between first and second positions. According toone embodiment of the invention, the first position is characterized bya mirror configured to reflect solar radiation toward the receiver, andthe second position is characterized by a second mirror or an opticallens being positioned adjacent the first mirror to direct a differentamount of solar radiation toward the receiver. The second mirror or thelens can be adjusted by rotating the first mirror, or by adjusting thesecond mirror or lens relative to the first mirror.

According to another embodiment of the present invention, the heattransfer fluid is circulated through the receiver and a solar collectionapparatus is adjusted between first and second positions. In the firstposition, a lens of the apparatus is configured to direct, i.e., byrefracting and concentrating, solar radiation toward the receiver andheat the heat transfer fluid therein. In the second position, the lensis configured to direct a second, lesser amount of solar radiationtoward the receiver. The apparatus can be adjusted by rotating the lensabout the receiver according to a relative position of the sun, and/orby adjusting a shade device to at least partially prevent thetransmission of solar radiation toward the receiver.

The receiver can be preheated before the fluid is introduced andcirculated by directing solar energy thereon while the receiver is in asubstantially empty condition. Further, fluid that is inadvertentlysolidified in the receiver can be melted. The heat transfer fluid, whichcan be a molten salt, can also be delivered to a power generator.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a schematic view illustrating a conventional trough-type solarpower plant;

FIG. 2 is schematic view in perspective illustrating a conventionaltrough-type solar collector;

FIG. 3 is a schematic view in elevation illustrating the solar collectorof FIG. 2;

FIG. 4 is a schematic view in elevation illustrating a solar collectorincluding a lens according to one embodiment of the present invention,shown in a first position;

FIG. 5 is a schematic view in elevation illustrating the solar collectorof FIG. 4, shown in a second position;

FIG. 6 is a schematic view in elevation illustrating a solar collectorincluding an adjustable lens according to another embodiment of thepresent invention, shown in a first position;

FIG. 7 is a schematic view in elevation illustrating the solar collectorof FIG. 6, shown in a second position;

FIG. 8 is a schematic view in elevation illustrating a solar collectorincluding two mirrors according to another embodiment of the presentinvention, shown in a first position;

FIG. 9 is a schematic view in elevation illustrating the solar collectorof FIG. 8, shown in a second position;

FIG. 10 is a schematic view in perspective illustrating a solarcollector according to yet another embodiment of the present invention;

FIG. 11 is a schematic view in elevation illustrating the solarcollector of FIG. 10, shown in a first position;

FIG. 12 is a schematic view in elevation illustrating the solarcollector of FIG. 10, shown in a second position;

FIG. 13 is a schematic view in elevation illustrating a solar collectoraccording to still another embodiment of the present invention, shown ina first position;

FIG. 14 is a schematic view in elevation illustrating the solarcollector of FIG. 13, shown in a second position;

FIG. 15 is a schematic view in elevation illustrating the solarcollector of FIG. 13, shown in a third position;

FIG. 16 is a schematic view illustrating a solar power system accordingto one embodiment of the present invention; and

FIG. 17 is a block diagram illustrating the operations for collectingsolar radiation according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, this invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Referring now to FIG. 4, there is shown an apparatus 100 for collectingsolar energy. The apparatus 100 is structured as a trough-type solarcollector, which includes a receiver 102 such as a pipe, tube, or otherfluid conduit or vessel that defines a passage 104 for receiving andheating the heat transfer fluid. The apparatus 100 also includes aconcave collector mirror 110 with a reflective surface 112 that isdirected toward the receiver 102 and configured to reflect solarradiation toward the receiver 102 to heat the heat transfer fluidtherein. A longitudinal axis of the mirror 110 can extend generallyparallel to an axis of the respective receiver 102, and preferably themirror 110 has a cross-sectional shape that is parabolic or nearlyparabolic so that light entering from a particular direction isconcentrated and reflected onto the receiver 102, which is located at afocus of the mirror 110. The mirror 110 can alternatively define otherconcave shapes for concentrating and reflecting light to the receiver102.

The size of the receiver 102 can correspond to the flow and heatrequirements of the apparatus 100 and a trough system in which thesystem is used. The receiver 102 can be larger, smaller, and define alarger or smaller area as required by the system. For example, in someembodiments, the receiver 102 can be less than about 1 inch in diameter.The receiver 102 can be formed of a variety of materials, preferably amaterial that can be operated for extended periods of time attemperatures of up to about 1100° F.

The receiver 102 typically includes a metal tube 103 in which the heattransfer fluid is circulated and a glass tube 106 that extendsconcentrically around the metal tube 103. A space 107 is defined betweenthe inner surface 108 of the glass tube 106 and the outer surface 109 ofthe metal tube 103, and the space 107 can be evacuated to increase theeffective insulation of the receiver 102. Solar radiation reflected fromthe mirror 110 is transmitted through the glass tube 106 to impinge onthe metal tube 103. The evacuated space 107 and, hence, the glass tube106, reduces the loss of heat from the metal tube 103 to the ambientenvironment due to conduction and convection therefrom. Further, themetal tube 103 can be painted or otherwise coated to increase theabsorption of solar radiation. For example, the metal tube 103 can bespray painted with black or other dark paint to increase the thermalabsorptivity thereof on the surface 109.

The collector mirror 110 can be supported by a base frame 114 or otherstructure, and can be adjustable relative to the frame 114. The mirror110 can be rotatably adjustable so that the mirror 110 can be rotatedabout the receiver 102 according to the relative position of the sun 50.For example, a drive mechanism and motor 120 or other actuator can beprovided for rotating the mirror 110. The drive mechanism and motor 120can be controlled by a controller 130, such as a computer, programmablelogic device, or the like. The controller 130 can communicate with asensor 132 or detector that determines the relative position ordirection of the sun 50, or the controller 130 can be pre-programmedwith a schedule of the position of the sun 50 such that sensing theposition of the sun 50 is not required. Thus, the mirror 110 can beadjusted continuously throughout various positions, e.g., so that themirror 110 is kept substantially perpendicular, i.e., aligned or normal,to the direction of the sun 50 and the solar radiation is reflected ontothe receiver 102, which is maintained at the focal line of the mirror110. Further, if multiple mirrors 110 are used for a single receiver, ormultiple collection apparatuses 100 are used for a single solar powergeneration system, each mirror or set of mirrors can be controlledindependently so that each receiver is heated as desired.

The apparatus 100 can also include an optical lens 140 for directingsolar radiation onto the receiver 102. As shown in FIG. 4, the opticallens 140 can be rigidly connected to one side of the mirror 110 so thatthe lens 140 is rotated with the mirror 110. As positioned in FIG. 4,the mirror 110 reflects light onto the receiver 102 to heat the receiver102. In another position, shown in FIG. 5, the mirror 110 and the lens140 are rotated such that the lens 140 is configured to intercept anddirect solar radiation onto the receiver 102, i.e., the lens 140 isdisposed between the receiver 102 and the sun 50. Thus, the mirror 110and the lens 140 can each be configured to heat the receiver 102 in therespective positions.

The lens 140 can be any of a variety of optical devices and can beformed of materials including glass, polymers, and the like. Accordingto one advantageous embodiment of the present invention, the lens 140can be a fresnel lens, i.e., a relatively thin lens defining a pluralityof grooves 142 on at least one of the surfaces. Fresnel lenses areavailable from a number of manufacturers including 3M Company of St.Paul, Minn. In some cases, the fresnel lens can be provided as a sheetor film of material that is cut and configured in the desired shape.

According to another aspect of the invention, the lens 140 can beadjustable relative to the mirror 110 so that the lens 140 can beretracted or otherwise stowed when not in use. For example, as shown inFIG. 6, the mirror 110 is configured to reflect solar radiation onto thereceiver 102, and the lens 140 is shown in a stowed position relative tothe mirror 110. Preferably, when the lens 140 is adjusted to the stowedposition, the lens 140 does not interfere with the reflection of solarradiation by the mirror 110 to the receiver 102. When it is desired touse the lens 140 for directing solar radiation onto the receiver 102,the lens 140 can be adjusted to an extended position as shown in FIG. 7.For example, an actuator such as a motor (not shown) can adjust the lens140 relative to the mirror 110. Further, the mirror 110 and the lens 140can both be adjusted by the motor 120 so that the lens 140 is disposedbetween the sun 50 and the receiver 102 and directs and concentratessolar radiation onto the receiver 102.

The lens 140 can be configured to direct less solar energy onto thereceiver 102 than the mirror 110. Thus, the lens 140 and mirror 110 canbe used to heat the receiver at different rates as desired duringsimilar solar conditions. For example, during a condition characterizedby a particular amount of solar radiation, the mirror 110 can be used toheat the receiver 102 at a relatively greater rate, or the lens 140 canbe used to heat the receiver 102 at a relatively lesser rate.

The apparatus 100 can alternatively include a first mirror 110 and asecond mirror 150 as shown in FIGS. 8 and 9. The first mirror 110, asdescribed above, can be concave, e.g., parabolic, and/or adjustable sothat the mirror 110 is configured to reflect solar radiation onto thereceiver 102. The second mirror 150 is also concave and configured toreflect solar radiation toward the receiver 102. For example, the secondmirror 150 can be parabolic and can define a linear focus that coincideswith the receiver 102. The first and second mirrors 110, 150 aretypically configured to concentrate and reflect solar radiation receivedfrom different directions to the receiver 102. Thus, the second mirror150 can be rigidly attached to the first mirror 110 such that themirrors 110, 150 can be rotated between first and second positions. Inthe first position, shown in FIG. 8, the first mirror 110 is configuredto reflect solar radiation onto the receiver 102 to heat the receiver102. In the second position, shown in FIG. 9, the first and secondmirrors 110, 150 are rotated such that the second mirror 150 isconfigured to direct solar radiation onto the receiver 102.

The second mirror 150 can be smaller than the first mirror 110, can havea different curvature than the first mirror 110, or can be lessreflective than the first mirror 110. Therefore, the second mirror 150is configured to reflect a lesser amount of solar energy onto thereceiver 102 than the first mirror 110. Thus, the first and secondmirrors 110, 150 can be used to direct different amounts of solarradiation toward the receiver for different modes of heating. Moreparticularly, the first mirror 110 can be used to heat the receiver 102at a relatively greater rate, i.e., with more thermal energy, and thesecond mirror 150 can be used to heat the receiver 102 at a relativelylesser rate, i.e., with less thermal energy.

During high power operation with the first mirror 110, no significantthermal energy is typically added to the receiver 102 by the secondmirror 150. Similarly, during lower power operation with the secondmirror 150, no significant thermal energy is typically added to thereceiver 102 by the first mirror 110.

The second mirror 150 can also be adjustable relative to the firstmirror 110. For example, the second mirror 150 can be extendable orotherwise adjustably connected to the first mirror 110. Thus, similar tothe configuration described above in connection with FIGS. 6 and 7, inwhich the lens 140 is adjustable to the mirror 110, the second mirror150 can also be adjustable relative to the first mirror 110 so that thesecond mirror 150 can be retracted or otherwise stowed, e.g., behind thefirst mirror 110, when not in use. Preferably, when the second mirror150 is adjusted to the stowed position, the second mirror 150 does notinterfere with the collection and reflection of solar radiation by thefirst mirror 110 to the receiver 102. Then, when it is desired to usethe second mirror 150 for directing solar radiation onto the receiver102, the second mirror 150 can be adjusted to an extended position usinga motor or other actuator. In addition, the first and second mirrors110, 150 can be adjusted in combination by the motor 120 such that theeither of the first or second mirrors 110, 150 is disposed relative thesun 50 to direct solar radiation onto the receiver 102. Advantageously,the retraction of the second mirror 150 can reduce wind loads thereon.Further, the amount of solar energy directed toward the receiver 102 canbe “fine tuned” by partially retracting or otherwise adjusting thesecond mirror 150 relative to the first mirror 110.

FIGS. 10-12 illustrate an embodiment of the present invention in which alens 160 is used without a mirror to heat the receiver 102 at variousrates, e.g., to selectively heat the receiver 102 at first or secondrates in response to the same amount of solar radiation. The lens 160can be a fresnel or other type of lens and can be similar to the lens140 described above in connection with FIGS. 4-7, e.g., defining grooves162 on at least one surface thereof. Preferably, the lens 160 is ofsufficient size for receiving, concentrating, and directing sufficientsolar radiation to provide heat to the receiver 102 at a desired rate.For example, the lens 160 can extend along the length of the receiver102 and can extend in an arc of about 130° around the receiver 102. Forexample, the arc defined by the cross-section of the lens 160 can have awidth W (FIG. 10) between about 2 and 20 feet. The lens 160 can besupported by a frame 168, which is rotatable or otherwise adjustablerelative to the base frame 114.

As illustrated in FIG. 10, the lens 160 can be adjustable to correspondto the relative position of the sun 50. For example, the lens 160 can besupported by the base frame 114 so that the frame 168 or other structureis rotatably adjustable and the lens 160 can be rotated about thereceiver 102 by a motor 166 or other actuator. As described above inconnection with the other embodiments of the invention, the motor 166can be controlled by a controller, such as a computer, programmablelogic device, or the like. The controller can be pre-programmed to trackthe sun and to communicate with a sensor or detector that determines therelative position of the apparatus 100 relative to the direction of thesun 50. Thus, the lens 160 can be adjusted continuously so that thesolar radiation is directed by the lens 160 onto the receiver 102.

The lens 160 can be adjusted between different positions in whichdifferent amounts of solar radiation are directed onto the receiver 102and, hence, the receiver 102 is heated at different rates. In a firstposition, the lens 160 is configured to maximize the heating of thereceiver 102. For example, the lens 160 can be positioned substantiallyperpendicular to the propagating direction of the solar radiation, asshown in FIG. 11, so that the solar radiation received by the lens 160is focused onto the receiver 102. Alternatively, in a second position,as shown in FIG. 12, the lens 160 is positioned at an oblique angle tothe propagating direction of the solar radiation. In the secondposition, only a small portion of the solar radiation is focused on thereceiver 102, such that the receiver 102 is heated less effectively thanin the first position. Thus, the rate of heating of the receiver 102 canbe adjusted by adjusting the position of the lens 160 according to therelative direction of the sun 50. Additionally, the transmitivity of thelens 160 can be reduced in the second position, e.g., by providing asemitransparent coating to a portion of the lens 160.

According to another embodiment of the present invention, the lens 160is fixed in position and configured to direct solar radiation toward thereceiver 102 regardless of the relative position of the sun 50 through arange of relative motion of the sun with respect to the lens 160. Forexample, as shown in FIGS. 13-15, the lens 160 can extend around thereceiver 102, e.g., in a parabolic arc of about 180° around the receiver102. Thus, the lens 160 can direct solar radiation toward the receiver102 regardless of the relative position of the sun 60. An adjustableshade 170 can be provided for controlling the amount of solar radiationthat is received by the receiver 102, the shade 170 being adjustablebetween two or more positions. For example, the shade 170 can berotatably mounted such that the shade 170 can be rotated about thereceiver 102 by an actuator 172 such as a motor. In a first position,shown in FIG. 13, the shade 170 does not substantially interfere withthe transmission of solar radiation to the receiver 102. In a secondposition, shown in FIG. 14, the shade 170 is configured between the lens160 and the receiver 102 and prevents the transmission of solarradiation between the lens 160 and the receiver 102. As shown in FIG.15, the shade 170 can be rotated to a third position in which the shade170 is positioned intermediate the first and second positions such thata reduced amount of radiation transmitted through the lens 160 isreceived by the receiver 102. Thus, by controlling the position of theshade 170, the amount of heating of the receiver 102 can be accuratelycontrolled.

The shade 170 can extend along the length of the lens 160, or multipleshorter shades 170 can be used for each lens 160. In either case, theshade 170 can define an arc that extends at least about as far aroundthe receiver 102 as the lens 160 so that the shade 170 can substantiallyblock solar radiation transmitted through each portion of the lens 160.According to one embodiment of the present invention, the shade 170 canalso define one or more semitransparent coatings, slits, holes, or otherapertures that allow radiation to pass through the shade 170 so that areduced and controlled amount of radiation is directed onto the receiver102 when the shade 170 is positioned between the lens 160 and thereceiver 102. For example, the shade 170 can define one or more slitsextending longitudinally such that the amount of radiation receivedthrough the shade 170 depends on the rotational position of the slitrelative to the sun 50.

The shade 170 can be formed to an arc shape, as illustrated, so that theshade 170 can be rotated between the lens 160 and the receiver 102. Forexample, the shade 170 can be formed of one or more sheets of steel. Inaddition, the shade 170 can be configured to be disposed proximate tothe lens 160, proximate to the receiver 102, or therebetween, when theshade 170 is rotated to a position for blocking solar radiation fromreaching the receiver 102. In other embodiments of the invention, theadjustable shade 170 can be used in conjunction with the adjustable lens160 discussed above in connection with FIGS. 10-12 or with the lensesand mirrors of any of the other embodiments of the present invention.Further, it is understood that the present invention can include one ormore of each of the lenses, mirrors, and shades.

Preferably, the apparatus 100 of the present invention is adjustable orotherwise configurable to control the amount of solar radiation that isreceived by the receiver 102. Thus, during any particular solarcondition, the apparatus 100 can be adjusted between first and secondmodes of operation in which different amounts of solar radiation arereceived by the receiver 102 and the receiver 102 is thus heated atdifferent rates. The apparatus 100 can be configured to receive amaximum amount of solar radiation in the first mode of operation and alesser amount of solar radiation in the second mode of operation. Thus,the first mode of operation can be used for heating the receiver 102 andthe heat transfer fluid therein during normal operation of the apparatus100. The second mode of operation can be used to deliver a lesser amountof thermal energy to the receiver 102, e.g., to preheat the receiver 102before the heat transfer fluid is introduced and circulated, to thaw, ormelt, heat transfer fluid that has solidified in the receiver 102, or tootherwise heat the receiver 102 and/or heat transfer fluid at a reducedrate, the purpose of which may be to preclude freezing.

Electrical resistant or other types of heating cable, wire, or tube canoptionally be mounted on the sunshade if required as an alternate meansof providing heat before circulation, to thaw or melt heat transferfluid that has solidified in the receiver 102, or to otherwise heat thereceiver 102.

Reference will now be made to FIG. 16, which illustrates a solar powersystem 200 according to one embodiment of the present invention. Thesolar power system 200 generally includes a fluid circuit 202 connectingat least one of the solar collection apparatuses 100 for circulating aheat transfer fluid therethrough so that the heat transfer fluid isheated in the apparatuses 100. It will be appreciated that the system200 can define a variety of configurations, FIG. 16 illustrating onlyone exemplary configuration. The fluid circuit 202 can be formed of anetwork of pipes, tubes, or other passage-defining components, and thesystem 200 can include components in addition to those shown, e.g.,reservoirs, heat exchangers, valves, and other devices for accommodatingand controlling the flow of the heat transfer fluid. For example, one ormore pumps 204 can be provided at various positions in the system 200for fluidly circulating the heat transfer fluid. Valves or other flowregulation devices can be used to control the flow of fluid through thevarious portions of the fluid circuit 202. Further, the operation of thesystem 200 can be controlled by a controller, such as the controller 130described above or an additional computer or other electrical processingdevice.

Various types of heat transfer fluids can be used with the system 200.According to one embodiment of the invention, the heat transfer fluid isa molten salt and, in particular, a nitrate salt including about 60%sodium nitrate and about 40% potassium nitrate. This nitrate salt isgenerally useful in a temperature range of about 550° F. to 1100° F. Attemperatures below about 450° F., the salt begins to solidify,increasing the resistance to flow. At temperatures above about 1100° F.,the salt begins to react chemically to form nitrites, which aregenerally more corrosive than the nitrate salt. Within the range of 450°F.-1100° F., the nitrate salt generally exists as a single phase, i.e.,a liquid, such that density of the fluid is substantially uniformthroughout the operation of the system 200. Alternative heat transferfluids include other liquid salts as well as oils and other fluids. Theheat transfer fluids can be selected according to the desired andanticipated temperature variation of the fluid in the system 200.

The fluid is circulated through the circuit 202 from a cold thermalstorage tank 230 to the solar collection apparatuses 100. Any number ofthe solar collection apparatuses 100 can be provided, and the collectionapparatuses 100 can be arranged in parallel and/or series configurationsso that the heat transfer fluid is heated contemporaneously and/orconsecutively in the different apparatuses 100. Each solar collectionapparatus 100 is structured to heat the heat transfer fluid circulatedtherethrough. For example, according to one embodiment of the presentinvention, the heat transfer fluid enters the solar collectionapparatuses 100 at a temperature between about 450° F. and 650° F., suchas about 550° F., and exits the collection apparatuses 100 at atemperature between about 950° F. and 1100° F., such as about 1050° F.

The system 200 can also include additional heating devices such as oneor more tower solar absorption device (not shown). For example, thetower solar absorption devices can be structured receive the heattransfer fluid from the trough-type solar collection apparatuses 100 andheat the fluid more efficiently to a temperature higher than thetemperature of the fluid in the solar collection apparatuses 100.Systems and methods for heating fluid according to such a two-stageoperation are described in U.S. application Ser. No. ______, entitled“Solar Power System and Method for Power Generation,” filed concurrentlyherewith, assigned to the assignee of the present application, and thecontents of which is incorporated herein in its entirety by reference.

The heat transfer fluid flows from the solar collection apparatuses 100to a hot thermal storage tank 250 and/or to a power generator 260. Thepower generator 260 can include a variety of devices for removing orotherwise using the thermal energy of the heat transfer fluid. Forexample, the power generator 260 can be a conventional steam generatorfor transferring thermal energy from the heat transfer fluid to aseparate fluid circuit holding water to thereby generate steam from thewater. The steam can be delivered to a turbine 262 to rotate the turbine262 and an electrical generator 264 coupled to the turbine 262. Acondenser 266 and cooling tower 268 can be used to condense and cool thesteam prior to preheat of water in preheater 267 and recirculation bypump 270, as is known to those skilled in the art. Thus, the thermalenergy of the heat transfer fluid can be converted to electrical energythat can be transmitted and distributed, e.g., by conventionalelectrical transmission devices and a distribution grid. In someembodiments, the temperature of the fluid is increased to about 1050° F.by the solar collection apparatuses 100, the water/steam is heated toabout 1050° F. by the power generator 260, and the water/steam is thencooled to about 550° F. in a conventional turbine that generateselectricity with an efficiency of about 40%. Alternatively, the powergenerator 260 can be a heat exchanging device that generates a hotworking fluid, such as steam, that can be delivered for heatingpurposes, such as for heating buildings or for industrial processheating. For example, the steam can be used as a heat source forconventional manufacturing processes or for water desalination.

The heat transfer fluid is cooled in the power generator 260 and pumpedto the cold storage tank 230 and the heat transfer fluid entering thesolar collection apparatuses 100, e.g., between about 450° F. and 650°F. such as about 550° F. The heat transfer fluid can be circulated backto the cold thermal storage tank 230 and/or recirculated through thesolar collection apparatuses 100. In some embodiments, the heat transferfluid is delivered from the power generator 260 to the solar collectionapparatuses 100 without passing through the cold thermal storage tank230 so that the heat transfer fluid can be reheated immediately. Inother embodiments, the heat transfer fluid can be stored in the coldthermal storage tank 230 before being reheated and re-circulated thoughthe system 200.

Generally, the system 200 can be operated with or without each of thethermal fluid storage tanks 230, 250. If one or more of the fluid tanks230, 250 are included in the system 200, each tank 230, 250 can beinsulated to minimize thermal losses to the environment. Some thermalenergy will likely be lost to the environment, though it is expectedthat the change in the temperature of the fluid due to such losses willbe minimal unless the fluid is stored for extended periods of time. Inone embodiment of the present invention, the heat transfer fluid isheated and delivered to the hot storage tank 250 and stored in the tank250 for subsequent power generation or reheating. For example, ifsufficient solar radiation is available for heating fluid in the solarcollection apparatuses 100, the fluid can be heated and stored and/orused in the power generator 260. If the solar collection apparatuses 100heat more fluid than that which can be accommodated by the powergenerator 260, some of the fluid can be directed from the solarcollection apparatuses 100 to the hot thermal storage tank 250.Similarly, if the solar collection apparatuses 100 heat more fluid thanthat which is necessary for generating the desired amount of power inthe power generator 260, the excess fluid can be stored in the tank 250.Subsequently, if the solar collection apparatuses 100 are unable to heatsufficient fluid for generating the desired power in the power generator260, the hot fluid from the storage tank 250 can be delivered to thegenerator 260. Thus, the hot storage tank 250 allows the timing of powergeneration to be shifted from the time of the absorption of solarenergy, thereby allowing power to be generated during night or otherperiods of relative darkness, during maintenance of the systemcomponents, and the like. Additionally, power can be generated accordingto the need for or current market value of electricity, or according toother factors.

The cold thermal storage tank 230 can also be used to receive fluid fromthe power generator 260 even if the solar collection apparatuses 100 arenot being operated. For example, if heat transfer fluid from the hotthermal storage tank 250 is used for operation of the power generator260 when the solar collection apparatuses 100 are not operated, such asat night or during maintenance of the devices, as described above, thefluid can be delivered from the power generator 260 to the cold thermalstorage tank 230.

Thus, the operation of the power generator 260 need not be restricted tothose times when solar energy is available for heating the fluid.Therefore, the system 200 is characterized by improved dispatchability,i.e., increased flexibility to dispatch power therefrom according to adesired schedule. In some embodiments, the solar collection apparatuses100 have sufficient capacity to heat enough fluid during a daytime whensolar energy is available such that the fluid can be used to operate thepower generator 260 during nighttime when solar energy is not available.Thus, the system 200 can be operated continuously during daytime andnighttime. In other embodiments, the power generator 260 can be operatedto generate power when power is required or when power prices are high,and operation can be slowed or stopped when power is not needed or whenpower prices are low. For example, the system 200 can operate to heatthe fluid according to the availability of sunlight, with power beinggenerated according to a current or anticipated desire for or marketvalue of electricity.

If there is insufficient solar radiation for heating the heat transferfluid in the solar collection apparatuses 100, such as at night or othertimes of darkness, the fluid can continue to circulate through thedevices or the fluid can be drained therefrom. For example, the heattransfer fluid can be drained from the solar collection apparatuses 100to the hot thermal storage tank 250 and/or the cold thermal storage tank230. Depending on the size and configuration of the solar collectionapparatuses 100, the fluid can be drained within a relatively shorttime, e.g., within a few minutes in some embodiments. The solarcollection apparatuses 100 can also be drained at other times when thesolar collection apparatuses 100 are not heating the fluid, such asduring times of maintenance and the like.

If the heat transfer fluid is drained from the solar collectionapparatuses 100, the solar receivers 102 can subsequently be preheatedbefore the circulation of the fluid is restarted. For example, the solarcollection apparatuses 100 can be adjusted to the second mode ofoperation in which a controlled amount of solar radiation is directedonto the receivers 102, thereby heating the receivers 102 at acontrolled rate. Thermocouples 210 or other temperature sensing devicescan be provided for detecting the temperature of the receivers 102. Forexample, as shown in FIG. 16, thermocouples 210 can be provided on eachsolar receiver 102 so that each thermocouple 210 is configured to detectthe temperature of the receiver 102. Each thermocouple 210 canelectrically communicate with a controller, such as controller 130, thatresponds to changes in temperature by controlling the configuration ofone or more of the solar collection apparatuses 100 according to thedesired amount of heating for the respective receiver 102. For example,if the receiver 102 of a particular solar collection apparatus 100 iscooler than the desired temperature for the receiver 102, the controller130 can increase the heating rate of the solar collection apparatus 100by adjusting the mirror, lens, and/or shade thereof. Alternatively, ifthe receiver 102 is warmer than the desired temperature, the controller130 can decrease the heating rate by adjusting the mirror, lens, and/orshade. In some cases, the receivers 102 can be preheated in asubstantially empty condition to a temperature that is approximatelyequal to the temperature of the heat transfer fluid that is subsequentlycirculated therethrough. Thus, the heat transfer fluid can be introducedinto the receivers 102 without causing a rapid thermal change andassociated stress in the receivers 102.

In other embodiments of the invention, the fluid can continue tocirculate through the receivers 102 during periods when the solarcollection apparatuses 100 are not heating the fluid or notsubstantially heating the fluid. For example, if solar radiation is notavailable due to nighttime and/or clouding or other weather oratmospheric conditions, the circulation of fluid can be continued untilsolar radiation is again available or a decision is made to interruptoperation. Heaters can be provided for heating the fluid in the storagetanks 230, 250, solar collection apparatuses 100, or other systemcomponents to maintain minimum operating temperatures throughout thesystem 200. If the fluid becomes cooler than a solidificationtemperature, the fluid will begin to solidify, or freeze, in theapparatuses 100. In some cases, the solidification of the fluid cancontinue until the passages 104 of the receivers 102 are blocked and thecirculation of the fluid therethrough cannot be continued. The fluid canbe melted by heating the receivers 102 when sufficient solar radiationis available. Preferably, the heating rate of the receivers 102 iscontrolled during melting of the fluid, e.g., by the controller 130 andin response to the temperature as measured by the thermocouples 210,which measure the temperature of the receivers 102 and, hence, the heattransfer fluid therein.

The receivers 102 can be controllably heated during preheating and whilemelting fluid therein. Further, the heating rate of each receiver 102can be coordinated with the receivers 102 of the other solar collectionapparatuses 100 so that the fluid is heated throughout the system 200 ata controlled rate, e.g., uniformly and/or according to a predeterminedrate or schedule. Thus, thermal stresses caused by nonuniform heatingcan be minimized, and the receivers 102 can be heated accurately to adesired temperature.

The operations for generating power using solar energy according to oneembodiment of the present invention are illustrated in FIG. 17. It isunderstood that some of the illustrated operations can be omitted fromthe method and additional operations can be performed without departingfrom the scope of this invention. Beginning at Block 300, it isdetermined if the receiver of the solar collection apparatus issubstantially empty of the heat transfer fluid. If the receiver issubstantially empty, it is next determined at Block 302 if the receiverrequires preheating. If preheating is required, the solar collectionapparatus is adjusted to a second position in which one or more mirrorsand/or lenses are configured to direct solar radiation toward thereceiver and thereby heat the receiver at a second rate. See Block 304.For example, one or more mirrors, lenses, and/or shades can be adjustedto direct the different amounts of solar radiation toward the receiversuch that the receiver is heated to a preheat temperature before thefluid is circulated therethrough. If the receiver does not requirepreheating, e.g., if the receiver is sufficiently warm to receive heattransfer fluid, the heat transfer fluid is circulated through a passagedefined by the receiver, and the solar collection apparatus is adjustedto a first position in which one or more of the mirrors and/or lensesare configured to direct solar radiation toward the receiver to heat thereceiver at a first rate, higher than the second rate. See Block 310.Returning to Block 300, if it is determined that the receiver is notsubstantially empty of the heat transfer fluid, it is next determined ifthe fluid in the receiver is at least partially solidified. See Block306. If the fluid is at least partially solidified, the solar collectionapparatus is adjusted to the second position to melt the heat transferfluid. See Block 308. Alternatively, if the heat transfer fluid is notsolidified in the receiver, e.g., if the fluid has been melted, the heattransfer fluid is circulated as indicated by Block 310. Further, theheat transfer fluid that is heated in the receiver can then be deliveredto a power generator such as a steam generator and electrical generator.See Block 312.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which thisinvention pertains having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. An apparatus for collecting solar energy, the apparatus comprising: areceiver extending along an axis and defining a passage for receiving aheat transfer fluid; a concave mirror configured to reflect solarradiation toward the receiver; and an optical lens positioned adjacentthe mirror, the lens being configured to direct solar radiation towardthe receiver, wherein the mirror and lens are configured to directdifferent amounts of solar radiation toward the receiver and therebyheat the receiver at first and second rates, respectively, duringsimilar solar conditions.
 2. An apparatus according to claim 1 whereinthe mirror is parabolic and configured to reflect solar energy towardthe receiver.
 3. An apparatus according to claim 1 wherein the mirror isrotatable about the axis of the receiver such that the mirror canreceive solar radiation from various directions and direct the solarradiation toward the receiver.
 4. An apparatus according to claim 1wherein the lens is adjustable relative to the mirror.
 5. An apparatusaccording to claim 1 wherein the lens and mirror are rigidly attached.6. An apparatus according to claim 1 wherein the lens is a fresnel lensstructured to at least partially refract light passing therethrough. 7.An apparatus for collecting solar energy, the apparatus comprising: areceiver extending along an axis and defining a passage for receiving aheat transfer fluid; and a first mirror being concave and configured toreflect solar radiation toward the receiver; and a second mirrorpositioned adjacent the first mirror, the second mirror being configuredto direct solar radiation toward the receiver, wherein the first andsecond mirrors are configured to direct different amounts of solarradiation toward the receiver and thereby heat the receiver at first andsecond rates, respectively, during similar solar conditions.
 8. Anapparatus according to claim 7 wherein the first mirror is parabolic andconfigured to reflect solar energy toward the receiver.
 9. An apparatusaccording to claim 7 wherein the first mirror is rotatable about theaxis of the receiver such that the mirror can receive solar radiationfrom various directions and direct the solar radiation toward thereceiver.
 10. An apparatus according to claim 7 wherein the first andsecond mirrors are rigidly attached.
 11. An apparatus according to claim7 wherein the second mirror is parabolic.
 12. An apparatus forcollecting solar energy, the apparatus comprising: a receiver defining apassage for receiving a heat transfer fluid; and a lens extendinggenerally parallel with the linear receiver, the apparatus beingadjustable between first and second positions, the lens in the firstposition being configured to direct solar radiation toward the receiverto thereby heat the heat transfer fluid in the passage at a first rate,and the lens in the second position being configured to direct a lesseramount of solar radiation toward the receiver than in the first positionto thereby heat the receiver at a second rate less than the first rate,during similar solar conditions.
 13. An apparatus according to claim 12wherein the lens is fixedly positioned and configured to direct solarradiation from a plurality of directions toward the receiver.
 14. Anapparatus according to claim 13 wherein the lens extends at least about170° around the receiver.
 15. An apparatus according to claim 12 furthercomprising a shade being adjustable between at least two positions tocontrol an amount of solar radiation received by the receiver andthereby control the amount of heating of the receiver, the shade in atleast one of the positions at least partially preventing the receptionof solar radiation that would otherwise be incident upon the lens. 16.An apparatus according to claim 12 wherein the lens is rotatable about alongitudinal axis of the receiver such that the lens can direct solarenergy from the sun toward the receiver throughout a range of relativepositions of the sun.
 17. An apparatus according to claim 12 wherein thelens is a fresnel lens structured to at least partially refract lightpassing therethrough.
 18. A method of heating a heat transfer fluidusing solar energy, the method comprising: circulating a heat transferfluid through a receiver defining a passage for receiving the heattransfer fluid; adjusting a solar collection apparatus to a firstposition in which a mirror is configured to reflect solar radiationtoward the receiver to thereby heat the receiver at a first rate; andadjusting the solar collection apparatus to a second position in whichan optical lens of the apparatus positioned adjacent the mirror isconfigured to direct solar radiation toward the receiver, to therebyheat the receiver at a second rate.
 19. A method according to claim 18wherein said first adjusting step comprises rotating the mirror aboutthe receiver according to a relative position of the sun such that themirror reflects solar radiation from the sun toward the receiver.
 20. Amethod according to claim 18 wherein said second adjusting stepcomprises adjusting the lens relative to the mirror according to arelative position of the sun such that the lens directs solar radiationfrom the sun toward the receiver.
 21. A method according to claim 18wherein said second adjusting step comprises adjusting both the mirrorand the lens, the lens being rigidly attached to the mirror.
 22. Amethod according to claim 18 wherein said second adjusting stepcomprises directing solar radiation onto the receiver in a substantiallyempty condition such that the receiver is heated to a preheattemperature before the fluid is circulated therethrough.
 23. A methodaccording to claim 18 wherein said second adjusting step comprisesdirecting solar radiation onto the receiver with the heat transfer fluidat least partially solidified therein such that the heat transfer fluidis melted.
 24. A method according to claim 18 wherein said circulatingstep comprises circulating a molten salt.
 25. A method according toclaim 18 further comprising delivering the heat transfer fluid to apower generator.
 26. A method of heating a heat transfer fluid usingsolar energy, the method comprising: circulating a heat transfer fluidthrough a receiver defining a passage for receiving the heat transferfluid; adjusting a solar collection apparatus to a first position inwhich a first mirror of the apparatus is configured to reflect solarradiation toward the receiver to thereby heat the receiver at a firstrate; and adjusting the solar collection apparatus to a second positionin which a second mirror of the apparatus positioned adjacent the firstmirror is configured to reflect solar radiation toward the receiver tothereby heat the receiver at a second rate.
 27. A method according toclaim 26 wherein said first adjusting step comprises rotating the firstmirror about the receiver according to a relative position of the sunsuch that the first mirror reflects solar radiation from the sun towardthe receiver.
 28. A method according to claim 26 wherein said secondadjusting step comprises adjusting the second mirror relative to thefirst mirror and according to a relative position of the sun such thatthe second mirror reflects solar radiation from the sun toward thereceiver.
 29. A method according to claim 26 wherein said secondadjusting step comprises adjusting both the first and second mirrorsaccording to a relative position of the sun such that the second mirrorreflects solar radiation from the sun toward the receiver, the first andsecond mirrors being rigidly attached.
 30. A method according to claim26 wherein said second adjusting step comprises reflecting solarradiation onto the receiver in a substantially empty condition such thatthe receiver is heated to a preheat temperature before the fluid iscirculated therethrough.
 31. A method according to claim 26 wherein saidsecond adjusting step comprises directing solar radiation onto thereceiver with the heat transfer fluid at least partially solidifiedtherein such that the heat transfer fluid is melted.
 32. A methodaccording to claim 26 wherein said circulating step comprisescirculating a molten salt.
 33. A method according to claim 26 furthercomprising delivering the heat transfer fluid to a power generator. 34.A method of heating a heat transfer fluid using solar energy, the methodcomprising: circulating a heat transfer fluid through a receiverdefining a passage for receiving the heat transfer fluid; adjusting asolar collection apparatus having a lens to a first position in whichthe lens is configured to refract solar radiation toward the receiverand heat the heat transfer fluid therein; and adjusting the solarcollection apparatus to a second position in which the lens isconfigured to refract a second amount of solar radiation toward thereceiver, the lens refracting less solar radiation toward the receiverin the second position than in the first position.
 35. A methodaccording to claim 34 wherein said first and second adjusting stepscomprise rotating the lens about the receiver according to a relativeposition of the sun such that the lens refracts solar radiation from thesun toward the receiver.
 36. A method according to claim 34 wherein saidsecond adjusting step comprises refracting solar radiation onto thereceiver in a substantially empty condition such that the receiver isheated to a preheat temperature before the fluid is circulatedtherethrough.
 37. A method according to claim 34 wherein said secondadjusting step comprises refracting solar radiation onto the receiverwith the heat transfer fluid at least partially solidified therein suchthat the heat transfer fluid is melted.
 38. A method according to claim34 wherein said first and second adjusting steps comprise adjusting ashade device of the apparatus to control an amount of solar radiationreceived by the receiver and thereby control the amount of heating ofthe receiver, the shade in at least one of the positions at leastpartially preventing the reception of solar radiation that wouldotherwise be incident upon the lens.
 39. A method according to claim 34wherein said circulating step comprises circulating a molten salt.
 40. Amethod according to claim 34 further comprising delivering the heattransfer fluid to a power generator.